Recently, a microneedle array has been frequently tried to be used as one of transdermal administration methods of a drug. Along with the popularization of a microneedle array, various attempts to fix a drug on a microneedle array have been made. In general, a microneedle array supporting a drug is prepared by dissolving a drug in a solvent and coating a microneedle array with the drug solution. However, when a microneedle array made of a biodegradable polylactic acid resin (PLA) or polyglycolic acid resin (PGA) is coated with a drug solution (e.g. an aqueous solution), it is essential to add an additive which acts as a polymeric tackifier to the solution to attach the drug on the surface of the microneedle array. For example, Patent Document 1 discloses that polysaccharides which are compatible with a drug (such as pullulan and hydroxypropylcellulose) can be used as a carrier for coating. Furthermore, Patent Document 2 discloses that preferred examples of a carrier for loading a drug on a microneedle array made of PLA include pullulan, carboxyvinylpolymer and the like.
Also, Patent Document 3 discloses that preferred examples of a carrier for loading follicle-stimulating hormone and the like on a microneedle array made of PLA include pullulan and sucrose which are compatible (i.e., uniformly soluble).
A carrier (i.e., an additive) used for coating a microneedle array is required to assure the same safety as that of an aqueous preparation for injection, and thus an additive available here is highly restricted. In many instances, the polysaccharides as shown in the above Patent Documents 1-3 are used as the carrier. Furthermore, the viscosity of a drug solution varies depending on the property of an additive used, and thus the amount of the loaded drug should be adjusted depending on the different viscosity. Also, as doing re-coating of a drug solution on a microneedle array, the drug attaches on the needle tip, which might cause the blunting of the needle tip of a microneedle array, and thus the puncturability of the microneedle array is supposed to be less.
Thus, in order to accurately inject a small amount of a drug into the skin along with maintaining the puncturability of a microneedle array, it is required to select a suitable additive. Although a variety of means for maintaining the puncturability of a microneedle array and effectively injecting a small amount of a drug such as a vaccine into the skin have been studied, any useful solutions have not been found yet.